I found this accidently when I dragged an email address out of its place on a web page. What bookmarking email addresses does is that, with one click, it opens a new message addressed to that email in your default email program. Hence, you could have a seperate emails folder for say, Apple support or whatever.

It's simple, just click and drag an email address from the webpage and drag it to the toolbar. You'll get a sheet asking you what you want to name it and where to put it.

[robg adds: You can also control-click on any email link on a page and select "Add Link to Bookmarks" from the contextual menu.]